In a conventional antenna duplexer, as shown in FIG. 29, a transmission filter 302, a phase shifter 303 (enclosed by dotted line), and a reception filter 304 are connected in this sequence between input and output terminals 301 and 305. And an antenna terminal 306 is connected between the transmission filter 302 and phase shifter 303. As the transmission filter 302, a surface acoustic wave (SAW) filter is used.
The antenna duplexer, upon being used for a portable terminal device, has been attempted to improve only its power durability against a signal input in the transmission filter 302 from the input terminal 301.
When the whip antenna of the portable terminal device, upon being extended completely and contracted completely, matches best in phase with the antenna duplexer. In other situation, for example, the antenna, upon not being extended completely or upon being broken, does not match in phase with the antenna duplexer. In an extreme case, ahead of the antenna terminal of the antenna duplexer may open.
In an antenna duplexer, loss or noise due to transmission signal running into a reception circuit have to be usually suppressed. Therefore, a phase shifter is provided so that the output of the transmission filter, that is, the reception circuit as seen from the antenna terminal may open in the transmission frequency band.
Under the condition that the antenna duplexer does not match in phase with the antenna, a part of an output signal from the output port of the transmission filter is reflected to the transmission filter. When state ahead of the antenna terminal opens, nearly whole the output signal enters from the output port to the transmission filter. In this case, a high-frequency voltage and a high-frequency current of about twice at maximum may be applied to the transmission filter by combination of an input signal and its reflected signal.
Therefore, in an antenna duplexer using a SAW filter as the transmission filter, a largest electric power is applied to a SAW resonator near the output port of the SAW filter. Accordingly, the SAW resonator close to the output port deteriorates.